1. Field of the Invention
The present invention relates to a magnetic refrigerator which has a magnetic shield between a magnet for generating a high-intensity magnetic field and a magnetic working material, and activates the magnetic working material demagnetized in an a almost-zero-intensity magnetic field to generate coldness.
2. Prior Art
Although a magnetic refrigerator which uses an adiabatic demagnetization method,, i.e., a conventional method for refrigeration to ultra-low temperature ranges has been used widely for research purposes, it has not been used in practice for industrial purposes. Such a magnetic refrigerator basically comprises a magnet generating a large magnetic field, a magnetic working material generating coldness in an adiabatic demagnetization process, a mechanism for repeatedly magnetizing and demagnetizing the magnetic working material and an adiabatic container for accommodating the magnetic working material.
Regarding the magnet of such a magnetic refrigerator, a superconducting magnet comprising a superconducting filament coil has been able to be used instead of a conventional electromagnet with an iron core. Compared with a magnetic flux density of about 2T generated by an electromagnet, a high-intensity magnetic field of 5T or more can be generated in the hollow section of a superconducting coil.
The magnetic working material is magnetized adiabatically in a high-intensity magnetic field and generates heat. In a zero-intensity magnetic field, the material is demagnetized adiabatically and generates coldness. Gadolinium-gallium-garnet or chrome alum is used as a material whose temperature changes greatly depending on the change in the magnetic flux density near the temperature of liquid helium. In addition, a variety of materials which are usable near the temperature of liquid helium are being examined.
Regarding the methods for transferring coldness which is generated by cooling the heat generated by the magnetic working material, various conventional methods, such as a method of using the circulation of gases or liquids, a method of using the heat conduction of solids and a method of using heat pipes, have been examined.
Regarding the means for repeatedly magnetizing and demagnetizing the magnetic working material, two conventional methods are available: a method for controlling the generated magnetic field itself by repeatedly turning on and off the current to an electromagnet or a superconducting coil and a method for repeatedly moving the magnetic working material from a high-intensity magnetic field range to a low-intensity magnetic field range in a constant magnetic field. The coil current on/off method is used to achieve a static magnetic refrigerator in which the magnetic working material is fixed. With this method, however, it is difficult to turn on and off large current. In the case of using a superconducting coil, large current also flows in the electric wires connected from the coil to an external power supply and in the external power supply itself when current is turned on and off. This generates Joule heat loss and reduces the heat efficiency of the refrigerator. This method is thus mainly used for small refrigerators. In the case of practically available magnetic refrigerators, a method of using a superconducting coil in the permanent current mode to maintain a high-intensity magnetic field at all times so that the magnetic working material itself is reciprocated or rotated in the hollow section of the coil or from the proximately of the opening of the coil to the distal section of the coil has been widely examined.
However, in the case of the above-mentioned method of reciprocating the magnetic working material, if the magnetic working material is moved to a completely-zero-intensity magnetic field, it is necessary to move the magnetic working material far away from the superconducting coil to a position wherein the intensity of the magnetic field is negligibly low. As a result, the reciprocating or rotating movement stroke of the magnetic working material must be increased. The size of such a refrigerator is required to be large while its refrigerating performance is rather low. In a conventional technology, the movement stroke is set at a practically satisfactory value. In this case, the demagnetizing process ends in the low-intensity magnetic field generated by the coil and the magnetic flux density of the magnetic working material is not zero. As can be clearly understood according to the magnetic Carnot cycle diagram, the refrigerating heat efficiency of the conventional refrigerator is lower than that of an ideal refrigerator which demagnetizes in a zero-intensity magnetic field.
As a conventional technology wherein the movement stroke of the magnetic working material is shortened and demagnetization is performed in a zero-intensity magnetic field, there is a known method, wherein a sub-coil disposed coaxially at the proximity of the main superconducting coil generating a high-intensity magnetic field for magnetization generates an opposite magnetic field which cancels the magnetic field generated by the main coil so that a zero-intensity magnetic field region is formed by the cancellation at a position very close to the opening of main coil.
In the case of the above-mentioned static magnetic refrigerator in which the magnetic working material is fixed, a type which uses the superconducting coil in the permanent current mode to magnetized and demagnetize the magnetic working material is the most favorable refrigerator since it requires no complicated movement means for the magnetic working material and no superconducting coil current on/off means and the energy efficiency of the refrigerator is superior. As a prior art which achieves this type of refrigerator, a refrigerator which magnetically shields and demagnetizes the magnetic working material by fixing the magnetic working material outside the opening of the superconducting coil and by using a magnetic shield provided reciprocatively between the superconducting coil and the magnetic working material has been disclosed in the Japanese Patent Publication No. 63-31716. The flat plane of the magnetic shield described in the publication has the shape of a small plate being smaller than the opening surface of the coil. Since the plate-shaped magnetic shield is smaller than the sectional area of the high-intensity magnetic field, no magnetic shield space is formed behind the magnetic shield. It is therefore almost impossible to demagnetize the magnetic working material. This case is explained as follows. If the plate has high-intensity magnetism, the magnetic lines of force simply permeate the plate, and if the plate is a superconducting plate, the magnetic lines of force pass around the plate to its rear side. In other words, a magnetic shield space can be formed behind a plate-shaped magnetic shield only when the surface area of the plate is sufficiently larger than the sectional area of the magnetic field generation source located ahead of the plate.
In addition, the static magnetic refrigerator requires a heat switch means which achieves efficient heat transfer between the fixed magnetic working material and heat and colds baths. A known conventional heat switch means is a type in which a crystal column is installed reciprocatively at the leading end of a copper heat conductor connected to a small gas cooler which functions as a heat bath, and the end surface of the crystal column faces an end surface of the magnetic working material so that they can closely contact each other. A low-temperature heat switch to be connected to the cold bath is available. The switch comprises a pipe covering the magnetic working material and connected to the cold bath so that a space is provided between the surface of the magnetic working material and the internal surface of the covered pipe to the extent that working gas does not cause convection in the space, and the liquid drops of the working gas generated by cooling the magnetic working material are accommodated in the cold bath.
The above-mentioned heat switch which functions by the close contact and separation of the crystal column and the magnetic working material requires an additional crystal column reciprocating means. Furthermore, the above-mentioned heat pump type comprising the pipe for covering the magnetic working material requires gas as a refrigerant and thus has disadvantages, i.e., slow action and low efficiency.